The use of ferrite particles to produce heating in alternating magnetic fields is known in the art. As disclosed in U.S. Pat. No. 3,391,846 to White, and U.S. Pat. No. 3,902,940 to Heller et al., ferrite particles have been used to produce heat where it is desired to cause chemical reactions, melt materials, evaporate solvents, produce gases, and for other purposes.
PCT International Publication WO84/02098 (Application No. PCT/US83/01851 corresponding to the U.S. Pat. No. 4,914,267) of Derbyshire discloses the use of ferromagnetic materials having the desired Curie temperature in electrically conductive layers to provide auto-regulated heating to the Curie temperature of the material upon application of an alternating current to the conductive layer of ferromagnetic material. The power applied to the ferromagnetic layer is in the form of an alternating current source which produces skin effect or eddy current heating in the continuous ferromagnetic layer. As the ferromagnetic layer reaches the Curie temperature, the permeability of the layer drops and the skin depth increases, thereby spreading the current through the wider area of the ferromagnetic layer until the Curie temperature is achieved throughout and the desired heating is achieved.
In my copending applications referred to above and incorporated herein by reference, I disclose the use of magnetic particles in combination with heat-recoverable articles in alternating magnetic fields to effect recovery. In connection with those systems, I have observed that it would be desirable to have a different form of self-regulating, self-heating articles which can be used more conveniently in some applications to heat various substrates and materials.
Therefore, it is an object of this invention to provide an improved article and an improved system for providing heat to substrates using alternating magnetic fields.